1. Field of the Invention
This invention relates to an insulated electric wire and a coil using that insulated electric wire.
2. Description of the Related Art
Generally, insulated electric wires for use for coils for electrical devices such as motors, transformers, etc. are required to be superior in electrically insulating properties, heat resistance, and mechanical properties such as flexibility and the like.
In recent years, motors have been reduced in size and weight. When a high power motor has been built into a small space, an insulated electric wire having a rectangular cross sectional shape has been widely used in order to increase a proportion of a cross sectional area of an electrical conductor in the insulated electric wire used in the small space.
In addition, along with high voltage driving for high power, inverter driving for power performance enhancement has been being developed rapidly. Because the motor is driven by a high voltage while at the same time being driven by an inverter, there is a high risk that a superposition of a high drive voltage and an inverter surge is likely to cause a partial discharge in the insulated electric wire of the motor, leading to dielectric breakdown.
For that reason, in recent years, in order to prevent the dielectric breakdown due to the occurrence of partial discharge, the insulated electric wire has been designed to have a high partial discharge inception voltage (PDIV).
It is known that the PDIV of the insulated electric wire is enhanced, for example, by lowering the dielectric constant of its insulation film. As the insulated electric wire with its insulation film having a reduced dielectric constant, Patent Document 1 for example discloses an insulated electric wire using in an electrically insulating layer a polyimide resin varnish having an imide group concentration of 15 to 20 percent after imidization of a polyimide precursor resin using a bisphenol A diphthalic acid dianhydride (BPADA) and a given aromatic diamine.